[GAMEPLAY] Targetable Karthus corpse
After an enemy Karthus dies and his passive is complete, the faint shadow left on the map (presumably as his corpse) remains targetable. By "targetable" I mean that holding alt and left-clicking on this corpse will ping to your team that you have "targeted" Karthus, even though he's dead.
I have not tested this on Summoner's Rift but have noticed every time that I play against a Karthus in ARAM.